Oriented polyethylene terephthalate film is a material that is affected at all stages of its life by the existence in the film of an inherent tendency to acquire a "set" on aging. The set may be referred to in the art as "core-set" when the set conforms to a core or tube on which the film has been wound and stored. The set can also occur in the absence of a core, for example when the film is rolled without a supporting core. The term "core-set" as used herein refers to both forms of set. Core-set may be explained as the result of plastic flow deformation imparted to self-supporting thermoplastic film when it is wound, and especially when it is wound on a core and stored at ambient conditions of temperature and relative humidity for a period of time sufficient for the film to acquire a substantially permanent curvature in the direction of its winding on the core. Core-set increases with increasing storage temperature and storage time and with decreasing roll diameter.
The problem of development of undesirable amounts of core-set in polymer films during storage in roll form is especially troublsome in the processing and use of photographic elements which comprise a high core-set-prone thermoplastic polymeric support material, especially polyethylene terephthalate. A high degree of curling tendency, or core-set-proneness, is particularly undesirable when the film element is intended to be used in that form, such as a microfiche. Such a film element, in its usual form, is a substantially flat piece of processed transparent photographic film usually measuring about 4" high and 6" wide, which bears projectionable microsized photographic images. Microfiche are widely used in storage and retrieval of information which can be projected and viewed on display panels or screens of readers or reader/printers. Efficient making, processing, storing, retrieving, reading and returning to storage of such small discrete film elements by high speed machines in trouble-free fashion require a high degree of flatness, or lack of core-set, in the film element.
It has been proposed to heat-temper a self-supporting film to reduce the core-set curling tendency of said film without distorting or shrinking said film by maintaining the film, while in the form of a stock or supply roll of film, for a period of from about 0.1 to about 1500 hours at a temperature ranging from about 30.degree. C. up to the Tg (glass transition temperature) of said polymer and an ambient relative humidity less than 100% until the core-set curling tendency of the film is reduced by at least 15%. This method is disclosed in U.S. Pat. No. 4 141 735. It suffers from the disadvantage that the reduction in curling tendency may not be sufficient in all cases, especially in those cases where the technique is used for controlling the core-set proneness of film that is wound in the form of strips onto small cores, such as 35 mm amatuer film wound on film spools with a core diameter as small as 12 mm.
It has also been proposed to control the curling tendency of a longitudinally stretched film, that already has a certain coiling tendency in one direction by coiling the film in the opposite direction onto a relatively small roll. If the film remains in the coiled-up condition for a considerable time, there is a tendency for the curl caused by the longitudinal stretching process of the film to be partially offset by the core-set. This technique is disclosed in GB-A-1 030 288. The method of the UK patent comprises the longitudinal stretching of a film by contacting on film surface in succession with one heated and one cooled roller, the other film surface being urged in contact with the temperature-controlling rollers by means of rubber-like nip rollers. This stretching technique is unsuited for the production of a polymeric film base for use as a support for photographic film, because the surface quality of suchlike film does not meet the stringent quality requirements of the photographic industry.
Further, there exists a process for making biaxially oriented photographic polyethylene terephthalate film having a deliberately introduced amount of longitudinal curl, comprising extruding molten polyethylene terephthalate polymer through a flat die onto a cooling drum, subjecting the film to molecular orientation by stretching the cooled down film longitudinally and transversely, and heat-setting the film, wherein the film is longitudinally stretched by applying longitudinal stretching forces to the film while heating the film, said heating comprising first pre-heating the film to increase the film temperature but not sufficient for plastic elongation to occur, and then stretch-heating the film to a temperature above the Tg (glass transition temperature)in a zone where the film is not supported by any roller, whereby rapid plastic elongation occurs under the stretching forces, and next rapidly and symmetrically cooling the film to a temperature below the Tg to arrest the stretching, which is characterized in that the stretch-heating of the film is performed asymmetrically temperature wise so that a temperature gradient "Delta T" exists across the film thickness, i.e. from one surface of the film to the other, which is larger than 10.degree. C., and that the longitudinal tension of the film during the stretching is less than 10 N/sq.mm.
This process is disclosed in EU-A1-0 299 560. If a film that has obtained a certain amount of longitudinal curl in accordance with this process, is coiled in the opposite direction onto a relatively small roll, as disclosed in GB-A-1 030 288 mentioned hereinbefore, there may be obtained film elements suitable as microfiches that show a reduced amount of curl, even after having been coiled into rolls for prolonged periods. The process according to EU-A1-0 299 560 does not permit, however one to obtain sufficiently large reductions of curling in the case of film wound on very small cores, such as 35 mm amateur film spools referred to hereinbefore.
Finally, it is known to make a film strip permanently coil-set for making a self-coiling film-sized sheet. This method is taught in U.S. Pat. No. 3,426,115.